Disposal of municipal waste is now becoming a serious problem due to the huge volume thereof which has been increasing from year to year.
Incineration is one of the ways for disposing the refuse and waste.
In the conventional burning apparatus or incinerator employing a fire grate, several problems have been encountered as summarized below.
(1) The plastics contained in the municipal waste melt and pass through the grate and are burnt around and beneath the grate whereby the grate is exposed to flame of high temperature and, thus, it is likely to be damaged at an early stage resulting in high maintenance cost. This problem is serious, since the proportion of plastics in the waste is relatively high and is increasing.
(2) Municipal waste comprises several kinds of materials which are not uniformly mixed and, therefore, respective constituents of the waste exhibit different burning characteristics which may create spots of extraordinarily high temperature within the incinerator during the incineration and, thus, tend to damage the wall of the combustion chamber. Also, such local high temperature is likely to generate NOx, thus, creating a problem of pollution.
(3) In the burning of solid materials, it is generally the heat-decomposing stage in the beginning thereof which requires heat-absorption and heat-transferring speed is relatively low thereby generally necessitating a fire grate having large surface area.
The fire grate is generally expensive due to its associated members such as means for effecting movement thereof and employment of a large grate naturally results in the necessity of a large incinerator and, accordingly, the expenses required for installation of the incinerator will be increased.
In order to solve the problems above, an incinerator of the fluidizing type has been utilized which employs inert solid particles as medium for forming a fluidized bed. The inert solid particles used are generally sand grains and, therefore, the term "sand grains" and/or "sand" is conveniently used in the present description in the sense that it is not limiting to "sand" but also covers any particles or grains suitable for forming the fluidized bed. The incinerator of the fluidized type is advantageous in the following points. (a) The liquified plastics or the like are heat-decomposed in the fluidized bed and burnt therein and, thus, possibility of damaging a gas diffusion plate corresponding to the fire grate in the apparatus touched upon above is obviated since such the plate is not subjected to the flame of the burning plastics. (b) Since the temperature in the fluidized bed is maintained substantially uniform, there is no problem derived from high temperature spots within the incinerator. (c) The heat transferring rate between the sand grains and the materials is quite large thereby making it possible to utilize a bed plate of relatively small surface area in the combustion chamber which makes it possible to have a compact sized incinerator thus lowering the installation cost of the apparatus.
The conventional incinerator of the fluidized bed type is, thus, quite useful for incinerating municipal waste or the like. However, it is necessary in such incinerators that the materials to be incinerated do not descend to the bottom of the fluidized bed. This is because, if the materials to be burnt decend to the bottom of the fluidized bed, the perforations in the diffusion plate will be closed whereby fluidization above the closed portion would be deactivated and sand grains in that portion will merge into a mass with the result that fluidization is insufficient or even formation of the fluidized bed becomes impossible.
Therefore, it is necessary to pulverize the materials to be incinerated in the apparatus of the fluidized bed type to below a certain particle size, usually with the range of from 50 mm to 70 mm. However, municipal waste or the like comprises several kinds of materials each having a different resistance to pulverization. For instance, brittle materials such as glass, potteries, ceramics and concrete are easily pulverized or crushed upon receiving impact thereon while tenacious materials such as rubber and plastics are not easily pulverized unless they are subjected to shearing action by use of sharp edges or the like. Therefore, if it is desired to pulverize the municipal waste which is a mixture of several kinds of refuse to grains reduced in size below a certain preferable value by a single apparatus, the apparatus suitable for such purpose must be capable of pulverizing as well as shearing. However, the edges required for shearing function are easily and quickly worn or damaged by the brittle materials such as glass, potteries, ceramics, concrete etc. or materials such as metals having high resistance against shearing action. Accordingly, it is difficult to pulverize municipal waste by a single apparatus to a degree satisfactory for incinerations in the conventional incinerator of the fluidized bed type.
Therefore, under such situation as above, in practice, the municipal waste is pulverized first by using an apparatus primarily adapted for pulverizing. Then the pulverized brittle materials are sorted out by sieving, etc. and the ferrous materials are magnetically removed. Thereafter, the remainder of the waste is shredded by another apparatus having sharp edges and primarily adapted for shearing and/or shredding functions. Therefore, the steps performed before the incineration have been more or less complex.
As explained above, the size reducing or regulating step before the incineration has been considered to be one of the most important problems to be solved when incineration of municipal waste is performed in a conventional incinerator of the fluidized bed type.
Therefore, it has been desired to simplify or eliminate the complex pre-processing required for incinerating the combustible constituents of municipal waste.